The subject matter disclosed herein relates to a semiautomatic transfer switch and, more specifically, to a transfer switch configured to connect a secondary power source to an electrical load upon verification of proper electrical connections between the secondary power source and the transfer switch.
Typically, utility power is delivered to the customer via a split-phase electrical distribution system. The split-phase electrical distribution system includes two “hot” wires, L1 and L2, which conduct alternating current having the same magnitude but offset by 180 degrees, and a neutral conductor, N. This split-phase electrical system is configured to supply power to alternating current (AC) loads of two different magnitudes. For example, 120 VAC loads are connected between either L1 or L2 and the neutral conductor and 240 VAC loads are connected between L1 and L2.
It is known that the utility power lines are exposed to harsh environmental conditions and can become inoperable for many reasons, such as inclement weather, ice, falling, trees, animal damage, etc., which may cause a portion of the power grid to fail or blackout. Consequently, many utility power customers utilize a backup generator to power some or all of the electrical loads present at the residence or building. Because the utility power is typically delivered via a split-phase electrical distribution system, the majority of backup generators are configured to deliver power in the same manner. However, backup generators are often installed by personnel not fully trained to install the equipment, for example, maintenance personnel or homeowners, increasing the potential for incorrect wiring of the backup generator.
Although the split-phase distribution system provides flexibility for the type of load to be connected, a fundamental hazard exists if the system is wired incorrectly or if a failure of a component or connection in the neutral conduction path causes the neutral conduction path to open. If the neutral conduction path is open and loads are connected to each of the two hot leads, current no longer returns on the neutral conduction path and a voltage divider network is established between the two hot leads. As a result, the voltage present between the two hot leads (e.g., 240 VAC) is divided proportionally between the two impedances seen in each half of the split distribution system. If the impedance of the loads present on one half of the system is significantly larger than the loads present on the other half, the majority of the voltage delivered by the utility will be present across the half of the system having the higher impedance. The electrical devices connected to that half of the system, which normally expect to receive a lower voltage potential (e.g., 120 VAC) will instead be connected to a substantially higher voltage potential, creating the potential to damage the electrical devices connected on the high voltage half of the system.